Looking Up: Stars are never where they’re supposed to be

If you are bothered by things out of place, such as a picture hanging on the wall at a funny angle, don’t let it spoil your view of the sky. Nothing is where it really us.

Peter Becker

If you are bothered by things out of place, such as a picture hanging on the wall at a funny angle, don’t let it spoil your view of the sky. Nothing is where it really us.

The sun rises every day before it really does, and it sets after it already did. The moon does the same thing. And the stars you see tonight aren’t necessarily where you are looking.

The sun and moon, as well as everything else cosmic, when seen along a flat horizon, is affected by Earth’s atmosphere and throws their image off. The air around Earth acts like a glass lens and refracts, or bends, the light of the heavenly object, make it appear in a different place than it really is.

We are pretty well assured that the sun will rise tomorrow. This is a wonderful constant in a world of change that we take so much for granted. The lens trick played by our atmosphere only adds to the beauty and drama of the sky. At the horizon, refraction causes the sun to appear a little more than its apparent width higher than it should be: 34’ of arc to be exact, just over a half degree. The effect of refraction decreases with altitude above the horizon; at 45 degrees above the horizon, the displacement is a mere 1’ of arc (1/60th of one degree). The value is zero, straight overhead (the zenith).

The whole electromagnetic spectrum -- which includes the entire rainbow of visible light -- is refracted at different amounts. Shorter wavelengths of blue and violet light are scattered away, and the longer wavelengths of orange and red are predominant. This gives the color of sunrise and sunset.

The bending of light at the horizon also leads to mirages and turbulence. Views of planets or the moon as seen in a telescope are usually superior when viewed higher in the sky. No telescope is needed to see the effect of atmosphere on the shapes of the sun and moon, when very low in the sky.

The November 2008 issue of Astronomy has a good article, “The day the Sun crawled,” by Stephen James O’Meara. A period of unusually warm air during the autumn can mean a temperature inversion in which warm, stagnant air is suppressing a layer of cooler air. A strange mirage can occur – it’s known as the “Novaya-Zemlya effect.” Dutch explorers first noted this effect in 1597 in the Arctic Ocean.

This causes the sun to appear to rise long before its predicted time. When observed in 1597, the sun was a full 5 1/2 degrees below the horizon when its image was seen over the water’s edge. O’Meara writes that he observed the phenomenon from a hill outside Boston in November 1991. He was looking for another, and elusive effect of refraction, a flash of green light briefly seen on the edge of the rising or setting sun.

Having studied where and when the sun was supposed to appear, O’Meara was startled to see the sun about 10 minutes too early and more than a degree too far east. Refraction actually reshaped the sun’s image into an orange, thin rectangle!

Speed of light

Stars and planets, as well as the moon and sun, are never seen exactly where they really are because of the great distances and time it takes their light to reach our eyes. Actually, this is true even with anything you see, even the hand in front of your face. The displacement of very close objects by the time it take light to reach you is so infinitesimally small, there is no practical application.

Light travels at 186,282.397 miles every second. Sunlight reaches us, on average, 1.282 seconds after reflecting off the moon. The average velocity of the moon in its orbit around Earth is .64 miles a second. By the time the moon’s light reaches your eyes, the moon has traveled, on average, 8/10th of a mile along in its orbit (.82 mile or 643 and a half feet). “Average” is stressed because the moon varies in velocity in its elliptical orbit, going fastest when closest to Earth, and likewise varies in distance and thus the time it takes for the light to reach you.

The effect is far greater for stars. The bright red star Aldebaran, in the constellation Taurus the Bull, for instance, moves at 33.4 miles an hour through the galaxy. The starlight takes 65 years to reach you, or about 569,400 hours. In the time it took the starlight to be seen, the star has moved 170,326 miles out of the way.

Constellations are imaginative pictures we make of the stars from our vantage point; the stars themselves are at widely different distances and are constantly on the move. Due to their great distance, however, the shape of a constellation would not begin to be noticeably but slightly different to the unaided eye for many thousands of years. Don’t throw your childhood star chart out just yet.

New moon is on Thanksgiving, Nov. 27. Watch Jupiter and Venus approach each other in the southwestern sky after dusk. On Nov. 30, they will be closest, only 2 degrees apart. Jupiter is at upper left.

Keep looking up!

Peter W. Becker is managing editor at The Wayne Independent in Honesdale, Pa. He has been an amateur astronomer since the age of 12, in 1969. He may be reached at pbecker@wayneindependent.com.